Conventional devices for generating mechanical power such as internal combustion engines, electric motors and gas turbines operate efficiently within a fairly narrow range of output speeds. Most vehicles, boats, elevators, machine tools, conveyors, generators, etc., have widely varying speed torque requirements. To date, no completely satisfactory device has been devised for accomplishing efficient matching of the speed and torque requirements between a motor and a load. Vehicles use either mechanical or hydraulic clutches with transmissions having gear ratios ranging from 2 to 18. Machine tools often use belt pulley transmissions. Machines, such as elevators, which require a continuous variable smooth transition from zero to full speed are limited to the use of D.C. motors. Finally, machines that require smooth variation between fixed lower and upper gear ratios commonly employ pulley belt drives which vary the effective pitch diameter of the pulleys. These, however, are bulky and are capable of handling only limited horsepower.
Hydrostatic drives employing a variable displacement pump and a fixed displacement motor are capable of providing precisely controlled infinitely variable ratios from zero to full speed at horsepowers up to 500. However, the maximum efficiency achieved by the presently known hydrostatic drives ranges from 70% to 75% at full load, seriously limiting their application. In many cases where the energy loss could be tolerated the cooling system required to dissipate the loss becomes too cumbersome and expensive.
The conventional hydrostatic transmission employs a variable displacement hydraulic pump, a fixed displacement hydraulic motor, and flow lines connecting the hydraulic pump to the hydraulic motor, a variable swashplate rotatably secured to an input shaft for controlling the stroke of the pump pistons and, therefore, the flow rate of the hydraulic fluid from the pump to the motor so as to cause the motor pistons to oscillate, and a fixed swashplate for converting the reciprocation of the motor pistons to rotation of an output shaft. In the neutral position, the variable swashplate is disposed perpendicular to the longitudinal axes of the pump pistons such that rotation of the input shaft and the variable swashplate does not cause the reciprocation of the pump pistons, and, therefore, no fluid flows to the hydraulic motor. As the angle of the variable swashplate is increased, the reciprocating stroke of the pump pistons correspondingly increases causing the motor pistons stroke to increase at the same rate. The increasing reciprocating stroke of the motor pistons causes the fixed swashplate and the output shaft to rotate at an increasing speed. When the angle of the adjustable swashplate is equal to the angle of the fixed swashplate, the output shaft rotates at the same speed as the input shaft. In this condition, there is a maximum amount of fluid flowing from the hydraulic pump to the hydraulic motor resulting in the maximum hydraulic losses, and correspondingly, minimum efficiency.